It is well known to provide a RF receiver in connection with a hearing aid. Such systems are often used in education situations where a hearing impaired student wearing a hearing aid receives a teachers voice through a RF transmission equipment. Where such systems are used in adjacent classrooms the transmission frequency must be different in the respective classroom in order to ensure receipt of the correct signal by the student.
One example of a hearing aid with a RF receiver is disclosed in CH 641619. The hearing aid with an RF receiver shown in this prior art document and other similar products available on the market today all comprise a single frequency receiving possibility. From U.S. Pat. No. 5,802,183 a further hearing aid is known which comprises the possibility of shifting between two frequencies, due to the presence of two crystals for determining the receiving frequency. In all of these previously known devices the frequency may be changed by changing the crystal element present for determining the receiving frequency. The very limited space available in such devices makes it difficult and often even impossible to incorporate a number of crystals corresponding to the desired receiving frequencies.
The change of a crystal is rather difficult due to the small size of these elements and the process is rather time consuming. Furthermore an amount of crystals corresponding to the number of desired frequencies is required for making the system operative under all desired circumstances. The device known from U.S. Pat. No. 5,802,183 offers the possibility of having two crystals and a switch for switching between the two frequencies. When however a larger number of frequencies is desired the same problem as described above exists.
The objective of the present invention is to provide a device, which offers the possibility of shifting between a larger number of frequencies than previously known, in a more efficient and less time consuming manner. A further objective is to provide a separate unit, which in connection with a hearing aid provides these same advantages.